I’d say my dog likes the scratching and doesn’t care at all about the leg movement. Do you think it’s particularly pleasurable for the dog when you hit the “sweet spot” and get the leg moving?
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I’d say they do. Dogs will tend to pull away from things they don’t like.
I always figured accessing the leg kick spot in dogs when rubbing their bellies produced the same sort of “pleasure” a human gets when the doctor hits their knee with a reflex hammer.
I don’t know if they do. I don’t get any hand-licking when I stop, or leaning into the scratching, unlike when I scratch in that spot right under the collar that they can never seem to reach. Based on that, I think they prefer collar scritches to the back scritches with leg kicking.
They’re very cute when I get the, “Why did you stop? That was great!” looks when I stop petting them.
Wait, what? We are supposed to like that?
I totally do. It’s the only reason I go to the doctor. Maybe it’s like catnip: doesn’t work for everyone.
OP I’ve done things to dogs that they didn’t like, or at least that caused them to express a keen interest in my stopping. Never had an objection to the belly scritch/kick start reflex.
I know my dogs do. Doxie actually smiles when you hit the spot. It’s great.
The best thing to do is to get your canine to start scratching his own belly with said leg. Voila, perpetual motion!
I had a dog that would often chase his tail and would rarely catch it. If it wasn’t for that dizziness, he was damn near a perpetual motion machine. 
I think they are telling us with the leg movement “yes, that’s the spot I can’t reach”
We asked this same question a while ago. Our dog seems to enjoy the belly/chest scratch plenty, and the leg spazzing is just a bonus: he seems to enjoy making us laugh at him doing that. Besides, as pointed out, if he did not like it he’d get up and move away.
I think the leg kick is incidental. They don’t dislike it, clearly, because they don’t leave or make you stop. But I don’t think it’s necessarily better than neck or back scratches that don’t cause kicking, either. I don’t think the leg kick says much either way, except “you’ve hit my involuntary reflex spot”.
I thought we might have, but searched and couldn’t find it.
I hope I am not opening a dark chapter in your life where you don’t leave your room, neglect your social responsibilities, and are stuck in an endless loop of self-gratification, but…
Do you know that you can do that reflex thing to yourself?
That’s disgusting.
Only with the lights off…
I’ve had a couple of dogs arrange themselves so that’s the spot I’m scratching. I suppose if a dog didn’t like it, he would move away.
Yeah, my Crew will move around while I’m scratching to make my hand go the spot they want. Teddy is worst/best at this.
I’ve heard the opinion that triggering this “leg spasm” response by scratching the spot on a dog’s flank is roughly comparable to holding down a small infant and tickling them. It’s maybe ok in small doses of a few seconds but not something that should happen more than once a week.
I’ve also heard the opinion that this action is somewhat masturbatory in it’s effect.
I can’t know the truth of either of these opinions. Either way, it’s not something that I choose to perform with any dogs I may interact.
My last two dogs didn’t have the spot. I think they were cats in disguise.
